Solar battery and method of making it



Oct. 6, 1964 c. A. ESCOFFERY SOLAR BATTERY AND METHOD OF MAKING ITOriginal Filed March 23, 1959 INVENTOR.

CHARLES A. ESGOFFERY ATTORNEYS FIG 2 FIG 4 Fla 5 Fla 7 FIG 8 UnitedStates Patent C) 3 151 379 SOLAR BATTERY ANn amnion on MAKING I'ICharles A. Escofiery, Los Angeles, Calif., assignor to InternationalRectifier Corporation, El Segundo, Caiif.,

a corporation of California Original application Mar. 23, 1959, Ser. No.801,234.

Divided and this application July 3, 1961, Ser. No.

6 Claims. (Cl. 2925.3)

This invention relates to solar energy converters, and more particularlyto such converters capable of developing relatively high voltage.

The principal object of the invention is to provide an effectivearrangement of a battery of solar or photovoltaic cells and a simple wayof assembling it.

Solar energy batteries or converters are well known. They commonlycomprise a number of photovoltaic cells connected together in a mannerto produce the desired overall voltage and current carrying capacity. Acommon form of a photovoltaic cell useful for this purpose comprises awafer of silicon of a selected conductivity type having diffused into asurface thereof a doping material or impurity of a character whichcreates in the surface region a conductivity of the opposite type. Thiscreates a PN junction near the surface. The action of light directed onsuch a surface generates a voltage in the region of the junction in awell known manner, this voltage being of the order of about a half voltper cell. A voltage much higher than that of a single cell is obtainableby connecting a plurality of such cells in series, which can be done byconnecting the sub-surface portions of the wafer of one cell with thesurface of the next cell in the battery of cells.

In accordance with the present invention I provide a unique andconvenient arrangement for a battery of such cells capable of exposing awide surface area to light, and I also provide a simple process formanufacturing the battery.

I carry out the process by fastening to a base support a relativelylarge area or wafer of semi-conductor material, preferably silicon,which has been made to have a desired conductivity type, that is, eitherthe N-type or the P-type. I form a surface layer on this area or waferof the opposite conductivity type from that of the remainder of thewafer which may readily be done by suitable diffusion at the surfacewith a desired doping material to produce such opposite conductivitytype. This operation of producing the layer of opposite conductivitytype is well understood; and it results in the formation of the desiredPN junction at the surface region.

In accordance with a feature of my invention I divide this relativelylarge area or wafer provided with the PN junction into a number ofindividual and electrically separated areas which form individual cells.I carry this out by applying a masking material having the area anddimensions of the desired individual cells; and this masking material isapplied in such a way as to leave a separation between all of theindividual areas of the masking material. By applying a suitabledissolving or etching substance to the masked surface, the material ofthe semiconductor area or wafer is dissolved out at the region betweenthe masked areas, thereby prodncing the desired individual cellselectrically unconnected with each other.

According to a further feature of the process I then remove some of themasking material from the surface of each individual cellular area, andthen apply a suitable etching or dissolving solution to these areas lastremoved, but only suflicient to remove the surface layer from each cellto expose the sub-surface or main wafer portion beneath.

To the main wafer portion I attach a suitable electrical connector orlead, and I also apply a suitable electrical connector or lead to thesurface area of each cell after removing the wax mask. This leaves theplurality bf individual cells each having a connector attached to itssurface area and a connector attached to its sub-surface region. Byconnecting the sub-surface region of one cell with the surface area ofanother, a series arrangement of cells can be made which will build upthe desired high voltage.

Preferably insulating material is placed between the in dividual cellsto prevent inadvertent short-circuiting from one cell to the next.

The foregoing and other features of the invention will be betterunderstood from the following detailed description and the accompanyingdrawing of which:

FIG. 1 is an isometric top view of a battery of photovoltaic cellsarranged according to this invention;

FIG. 2 is a cross-section view taken at line 22 of FIG. 1 showing aportion of the area of the battery as it is being constructed anddepicting a semi-conductor wafer or layer on a base support and having asurface layer of the opposite conductivity type from that of the rest ofthe wafer;

FIG. 3 shows the same view as FIG. 2 and also an arrangement of maskingareas on the upper surface;

FIG. 3a is a fragmentary face view of a part of the structurerepresented in FIG. 3;

FIG. 4 shows the view of FIG. 3 but with the wafer material in theregions between the masking areas removed;

FIG. 5 shows the view of FIG. 4 but with some of the masking material ofeach masking area removed;

FIG. 5a is a fragmentary face View of a part of the structurerepresented by FIG. 5;

FIG. 6 shows the same view as FIG. 5 but with the upper surface of thesemi-conductor material removed at each area where the masking materialhas been removed;

FIG. 7 shows the same view as FIG. 6 but with all of the masking areasremoved; and

FIG. 8 shows the same view as FIG. 7 but with electrical connectingelements attached to each cell with provision for connecting one cellwith the next.

Referring to FIG. 1 of the drawings, there is shown a base or mountingsupport It) of electrical insulating material which may be glass orporcelain or other ceramic of high purity and which may have a square orrectangular shape, as shown. There are fixed on the base a plurality ofphotovoltaic cells 11 side by side over the surface of the base andseparated from each other by insulating material 12. Each cell 11comprises a wafer or layer 13 of semi-conductor material, preferablysilicon, which is selected to have a desired type of conductivity, suchas the N-type conductivity. Each of these wafers is shown of square orrectangular shape so that a large number of the wafers will convenientlyfit the square or rectangular shape of the base panel 10. In accordancewith a practice common in the making of such photovoltaic cells, theupper surface 14 of each cell is made to have a conductivity of theopposite type from that of the rest of the wafer semi-conductor materialof the cell. Thus, if the main wafer of the cell is of the N-typeconductivity, the surface layer 14 will be made to have the P-typeconductivity so that a P-N junction is formed at theinterface region ofthe Wafer and its surface layer 14. As is well known, the efifect oflight, such as sun light, directed on the surface layer 14 generates avoltage at the PN-junction. The surface layer 14 of each cell is .;madeless than fully coextensive with the area of the wafer 13. Thus, alongthe right-hand margin of each cell 11, as seen in FIG. 1, there is astrip 15 which is not covered by the surface layer 14, but which insteadleaves the main semi-conductor material of wafer 13 exposed. Along thisexposed margin of each wafer 13 there is attached or affixed a strip 16of electrical conducting material making good electrical connection withthe respective Wafer 13. The strip 16 constitutes one of the terminalsof the respective photovoltaic cell 11. The other terminal of each ofthe cells is provided by another strip 17 attached or afiixed to thesurface layer 14, and the strip 17 is located at the opposite side ofthe cell from the terminal strip 16 of the cell.

All of the photovoltaic cells of FIG. 1 are shown connected in series bymeans of electrical connectors 18 each of which extends from a terminalstrip 16 of one cell to the adjacent terminal strip 17 of the nextadjacent cell, and by connectors 18a which extend from a terminal strip16 of a cell at the end of one row of cells to the terminal member 17 ofthe end cell of the next row. This battery of series connectedphotovoltaic cells then develops its full voltage across outputterminals 19 and 20, the terminal 19 being connected to the terminalstrip 17 in the end cell of the row of cells at one end of the panel 14and the terminal 20 being connected to the terminal strip 16 of the endcell in the row at the opposite end of the panel 10.

A preferred method of making this battery in accordance with the presentinvention is illustrated in FIGS. 2 to 8 which show a cross-sectiontaken at line 2-2 of FIG. 1 and showing the material and constructionassociated with three adjacent ones of the cells in the row at the line2-2. Thus, there is applied and afiixed to the base panel 10 a solid anduniform layer of the semi-conductor material 13 which is preferablysilicon, although it should be understood that other semi-conductormaterial such as germanium could be used instead. The semiconductormaterial may be applied in a suitable manner. A preferred way is toapply it to the panel by depositing it in a vapor phase. Silicon, forexample, can be deposited by passing a suitable silicon compound, suchas silicon tetrachloride in vapor phase over the base panel 10,preferably a material such as a high-purity ceramic .or porcelain whichfor this purpose is heated. The silicon will deposit from the siliconcompound on the panel under this condition in a manner describedhereinafter in greater detail; and to give the silicon its desired typeof conductivity suitable doping material is included in the vapor. Thus,to produce N-type silicon on the panel, a doping material such asarsenic may be used.

Following this, the surface .layer 14 is applied, which in the case ofN-type silicon for the layer 13, may be done by diffusing into the uppersurface a substance such as boron, aluminum, gallium or indium, whichwill cause the silicon of layer 14 to be of the P-type. When boron isused for the doping material, for example, the boron ditfusion caneffectively be carried out in a well known manner by application ofboron trichloride to the silicon surface at a high temperature, forexample, around 1000 C.

Following the application of the surface layer 14, the size and shape ofthe ultimate individual cells are then established by a maskingoperation. This is accomplished by applying to surface 14 a suitablemasking material 21 (FIGS. 3 and 3a) coinciding in dimensions with thedimensions of the cell. Thus, if each individual cell is to be arectangle one-half centimeter by two centimeters, for example, this willbe the dimensions of each of the masking areas 2.1. The masking materialmay be any of the Well known materials used for such purposes which isadherent to the semi-conductor, such as for example, awax of which anyof a number may be used, for example bees wax or parafiin or a suitablesealing wax, or the like. Each of these coverings of masking material2.1 will have spaces 9 around them to separate them from each of theother adjacent masking areas, as shown in FIGS. 3 and 3a. An etchingsolution is then appliedto the surface, and the solution shouldbe onewhich readily attacks the semi-conductor material and does not readilyattack the masking material.

A suitable etching solution for use with silicon semiconductor is anacid such as hydrochloric acid or nitric acid or a mixture ofhydrochloric and nitric acids. By this treatment the semi-conductor willbe dissolved or removed at the regions between the masking areas 21 toexpose the base panel 10 and thereby cause the spaces 9 to form down tothe panel 10 as shown in FIG. 4. This will produce the desiredseparation of the individual semiconductor areas from each other toconstitute the separate cells. The masking material 21 is then removedalong a side of each cell area to uncover part of the surface layer 13of the semi-conductor, this uncovered portion being the area 15 of eachcell shown in FIGS. 5 and 5a. The removal of the masking material fromover the regions 15 may be accomplished for example by dissolving it ina solvent for the masking material, while protecting the remainder ofthe masking material by a suitable protective coating or mask of its ownwhich is not so dissolved. It is noted that in one row of cells theareas 15 are located along the right side of the cell area (as seen inFIG. 5a) and in the next adjacent row the areas 15 are located along theleft side of the cell area.

a base, such as base it the silicon will deposit on the Following this,the etching solution is again applied, but only sufficiently to removethe surface layer 14 of the semi-conductor to leave the mainsemiconductor body 13 exposed at the areas 15, as shown in FIG. 6. Thenthe masking material is removed which can be done in the case of a waxmasking material by dissolving in a solvent such as trichlorethylene ora chlorinated hydrocarbon or the like. This will leave the structure inthe form shown in FIG. 7. Following this, the spaces 9 between theindividual semi-conductor elements 13 are preferably filled with theinsulating material 12 to prevent short circuits. This material 12 maybe a suitable insulating resin or varnish preferably of high qualitywhich can bereadily applied in liquid or fluid form and allowed tosolidify.

The cells are now ready to be series connected, which is done byconnecting the P-type region of one cell with the N-type region of thenext. For this purpose the strips 16 and 17 shown in FIG. 1 are applied.The strips for the P regions may be aluminum and the strips for the Nregions may be antimony or metals andalloys containing antimony, orarsenic, or phosphorus. Thus strips 16 may be antimony and strips 17aluminum. These strips may be applied by depositing them in a vacuumfrom the respective metals in vapor phase. Since it is difiicult toapply heavy deposits of aluminum by vacuum evaporation, the aluminumstrips may, if desired, first be cut to size and then fused in place;and the antimony or metals and alloys containing antimony, or arsenic,or phosphorus could be similarly applied if desired.

Then the connectors 18 are applied which can be done by fusing them asmetal strips to the strips 16 and 17. A preferred way is to form theconnectors 18 by evaporating a metal such as silver in vacuum anddepositing it as the strips 18, in which case they would rest on theinsulation 12 and also connect with strips 1-6 and 17. The strips 18a,19 and 20 may be metal strips fused to the respective terminals.

Although the material of wafer or layer 13 has been suggested as N-typesemi-conductor it will be understood that it may, if desired, be ofP-type semi-conductor instead. The choice of the N or P-type is made, asis well known, by the selection of doping material or impurityincorporated into the semi-conductor. For example, to make the material13 of N-type silicon the silicon can be doped with an element of Group Vof the periodic table, such as arsenic, and if on'the other hand it isdesired to make the material 13 of P-type silicon, the silicon can bedoped with an element from GroupHI of. the periodic table, such asboron. It follows that if'semi-conductor 13 is N-type, its surfacelayerld will be P-typeywhich can be accomplished by doping the siliconat the surface with a Group III element. If layer 13 is to be P-type,then the surface layer will be N-type which can be accomplished bydoping the surface with a Group V element.

Regardless of whether it is the N or the P-type, the substance 13 ispreferably made very thin, such as the order of few thousandths inch,for example .0005 to .0030 inch in thickness. In some cases it may bemade much thinner, such as the order of .0001 to .0002 inch thick, whichis about the same order of thickness as the thickness which is desiredof surface layer 14.

It will be recognized that the thicknesses of layers and material shownin the drawings are not in proportion to their actual thicknesses. Ingeneral, for ease of illustration, in the drawings the various parts areshown proportionately much thicker than they will actually be made.

The wafers or layers 13 and 14 can be formed in any of a number of ways,depending somewhat on the size of the area and other factors. It wouldbe desirable from the standpoint of conversion efficiency to make thesemiconductor a monocrystalline structure, such as a single siliconcrystal. It may be possible to do this, particularly where the surfacearea is not too large. But it may be easier instead, to provide apolycrystalline material 13, which may be done in any of a number ofways such as powder metallurgy or vacuum evaporation or a vapor phasedeposition. These several techniques have heretofore been used inconnection with semi-conductor materials.

In the powder metallurgy technique, the silicon is used in the form offinely divided particles or powder doped with a suitable material tocreate either the N or P-type silicon, and the powder be compressed ontothe base at a high pressure such as the order of 25,000 pounds persquare inch at an elevated temperature such as 900 to 1200 degreescentigrade. This will cause sintering of the silicon particles andattendant coalescing, after which the silicon is annealed in a suitableoven at a high temperature which may be somewhat less than 900 C. Thiswill result in a polycrystalline structure for the semi-conductor 13,and it will be adherent to the base. To provide the surface layer 14 ofthe opposite conductivity type, a suitable doping material can beimparted to this surface layer 14. For example, if material 13 is of theN-type silicon, its surface 14 can be converted to the P-type by passinga compound such as boron trichloride over it in vapor form at hightemperature, which will cause some boron to enter the surface layer 14to create the P-type silicon.

In the vacuum evaporation technique the process can be similar to thatused by industry for many metals and materials. To adapt it to silicondeposition, the silicon is placed in a boat or crucible in an evacuatedcontainer and heated to evaporate the silicon. The base, such as thebase 10 on which the silicon is to be coated, will be placed in theevacuated container so that the evaporated silicon deposits on it andadheres to it; and the desired doping material will be present to causethe silicon to take its desired N or P-type conductivity. After thematerial 13 is thus applied, the surface layer 14 of the oppositeconductivity type will be formed in any desired manner as mentionedabove, for example by exposing the surface 14 to the appropriate kind ofdoping material either in the vacuum or out of vacuum.

The vapor phase deposition technique is carried out in a known manner ofdepositing silicon from a suitable silicon compound; and this proceduremay be used to deposit the substance 13 and cause it to be adherent toits base. It can be done for example by reduction of silicontetrachloride with zinc vapor, or by hydrogen reduction of silicontetrachloride, silicon tetrabromide or trichlorosilane; or again it maybe done by thermal decomposition of silicon tetraiodide or of silane. Bypassing a gas of a silicon compound selected from the above group overterial will be supplied to provide the desired N or P-type conductivity.

base. As in the other processes, a suitable doping ma 0 In the appendedclaims, for the cake of definition, the semi-conductor material 13 isgenerally referred to as a wafer even though it is usually very thin;and the surface 14 is referred to as the surface layer.

The battery can be made with almost any desired number of cells, forexample, I00 to 300 cells; and it is entirely practical to obtain outputvoltages of 25 to volts. The dimensions of the individual cells may beas desired in accordance with the desired current output. The area ofeach individual cell may for example be of the order of one centimeterby one centimeter, or one-half centimeter by two centimeters or one-halfcentimeter by six centimeters, or the like.

While the invention has been described with particular reference tosilicon as the semi-conductor material, it should be understood thatother semi-conductor material may be used instead if desired. Forexample, it may be possible to use germanium, and the formation ofgermanium P-N junctions is well known. Also it may be possible to usecadmium sulphide or gallium arsenide instead of silicon.

It will be recognized that by my invention there is provided aneflicient and relatively inexpensive form of solar converter which canbe adapted to a wide range of volt-. age and current. By the simple stepof connecting suflicient cells in series desired values of outputvoltage may be obtained. It will be understood that in order to build upthe current output capacity the individual cells may be madesufiiciently large, or else cells may be connected in multiple. Thearrangement adapts itself readily to any desired combination of seriesand pmallel arrangement to obtain the desired voltages and currents.

This application is a division of my copending application, Serial No.801,234, filed March 23, 1959, and en tilted Solar Battery and Method ofMaking It (now abandoned).

It will be recognized that modification Within the scope of theinvention may suggest themselves to those skilled in the art, and theinvention is not limited except in accordance with the scope of theappended claims.

What is claimed is:

l. The method of making a solar energy converter which comprises:

(a) applying a coating of semi-conductor material of one of theconductivity types on an insulating base,

([1) doping the surface of the semi-conductor material to create asurface layer of the conductivity type oppositethat of thefirst-mentioned conductivity type,

(c) applying masking material over the surface layer at separated areaswhich define the areas of individual cells,

(d) removing the surface layer and the coating of semi-conductormaterial at the regions between said separated areas of maskingmaterial,

(6) removing the masking material from a part of each of said separatedareas,

(7) removing the surface layer at each of the lastmentioned parts toexpose the subsurface semi-conductor material thereat,

(g) removing the remainder of the masking material,

and

(h) applying a first terminal strip to the surface layer of each celland a second terminal strip to the exposed subsurface semi-conductormaterial of each cell.

2. The method as defined in claim 1, including the steps of applying aninsulating material to the insulating base in the regions whereat thesurface layer and the coating of semi-conductor material have beenremoved in step (d) and electrically connecting the first and secondterminal strips of a plurality of the adjacent cells.

3. The method as defined in claim 1, in which the conductor material issilicon.

4; The method as defined in claim 1, in which the semisubsurfacesemi-conductor material is P-type silicon and the surface layer thereofis N-type silicon.

5. The method as defined in claim 1, in which the'first terminal stripis constituted of aluminum and the second terminal strip is constitutedof a material selected from the group consisting of antimony, arsenicand phosphorus.

6. The method of making a solar energy converter which comprises:

(a) vapor depositing a silicon semi-conductor body of P-typeconductivity on an insulating base;

(12) ditlusing boron trichloride into the upper surface of said body tocreate an N-type silicon surface layer;

(0) applying a wax masking material over the N-type silicon surfacelayer at separated areas which define the areas of individual cells;

(d) etching the N-type silicon surface layer and the P-type silicon bodywith an acid etchant to expose the surface of the insulating base in theregions between said separated areas of said Wax masking material;

(2) dissolving the Wax masking material from a part of each of saidseparated areas;

(f) etching the N-type silicon surface layer at each of thelastrmentioned parts with an'acid etchantto ex pose the aligned parts ofsaid P-type silicon semiconductor body;

(g) dissolving the remainder of the Wax masking ma terial;

(h) applying an insulating coating to the insulating base in the regionsbetween said separated areas;

(i) vapor depositing a first terminal strip. constituted of aluminum onthe exposed P-type silicon semiconductor body parts, and a secondterminal strip constituted of a material selected from the groupconsisting ofantimony, arsenic and phosphorus on the remaining N-typesilicon surface layer of each cell; and

(i) vapor depositing metallic connectors between the first and secondterminal strips of the adjacent cells.

References Cited in the file of this patent UNITED STATES. PATENTS

1. THE METHOD OF MAKING A SOLAR ENERGY CONVERTER WHICH COMPRISES: (A)APPLYING A COATING OF SEMI-CONDUCTOR MATERIAL OF ONE OF THE CONDUCTIVITYTYPES ON AN INSULATING BASE, (B) DOPING THE SURFACE OF THESEMI-CONDUCTOR MATERIAL TO CREATE A SURFACE LAYER OF THE CONDUCTIVITYTYPE OPPOSITE THAT OF THE FIRST-MENTIONED CONDUCTIVITY TYPE, (C)APPLYING MASKING MATERIAL OVER THE SURFACE LAYER AT SEPARATED AREASWHICH DEFINE THE AREAS OF INDIVIDUAL CELLS, (D) REMOVING THE SURFACELAYER AND THE COATING OF SEMI-CONDUCTOR MATERIAL AT THE REGIONS BETWEENSAID SEPARATED AREAS OF MASKING MATERIAL, (E) REMOVING THE MASKINGMATERIAL FROM A PART OF EACH OF SAID SEPARATED AREAS, (F) REMOVING THESURFACE LAYER AT EACH OF THE LASTMENTIONED PARTS TO EXPOSE THESUBSURFACE SEMI-CONDUCTOR MATERIAL THEREAT, (G) REMOVING THE REMAINDEROF THE MASKING MATERIAL, AND (H) APPLYING A FIRST TERMINAL STRIP TO THESURFACE LAYER OF EACH CELL AND A SECOND TERMINAL STRIP TO THE EXPOSEDSUBSURFACE SEMI-CONDUCTOR MATERIAL OF EACH CELL.